Without limiting the scope of the invention, its background is described in connection with pulsatile jet flow.
Pulsed jets have found application in a variety of situations ranging from erosion and breaking of solid components in drilling applications (e.g., U.S. Pat. Nos. 4,607,792, 4,681,264, and 4,389,071) to propulsion. In studies related to propulsion, results for fixed/tethered jets have shown that jet pulsation may be used to augment thrust over equivalent steady jets (Choutapalli, 2006; Krueger and Gharib, 2005). Additionally, studies of vehicles and aquatic animals propelled by pulsed jets have shown that short pulses producing isolated vortex rings have higher propulsive efficiency than longer jet pulses (Bartol et al., 2008; Bartol et al., 2009b; Nichols, et al., 2008). Thus, in a variety of applications involving jet flow, it may be advantageous for the jet flow to be pulsed.
Numerous means for pulsing jet flows have been proposed and implemented in the scholarly and patent literature. One common method is to use transient piston motion to effect jet pulsations. For example, a piston situated in a plenum (as in Krueger and Gharib, 2005) may move forward in short steps to eject fluid slugs from a nozzle. Alternatively, the piston may oscillate back and forth with the direction of fluid motion governed by check valves (as in Nichols, et al., 2008) so that forward translation of the piston ejects a fluid slug from a nozzle while no net fluid motion occurs during piston retraction. Similarly, in U.S. Pat. No. 4,607,792 a liquid jet pulse is ejected into air by the forward motion of a piston and the plenum is recharged with liquid upon retraction of the piston. Cycling the piston motion generates a pulsed jet.
Another common method for creating jet pulses is to “shutter” a primary jet. In U.S. Pat. No. 3,883,075, pressurized flow is directed to a rotating nozzle block containing nozzles at fixed angular locations around the block. As the nozzle block is rotated, a jet pulse is released every time a nozzle aligns with the flow supply. Similarly, Choutapalli (2006) uses a spinning disk with holes to “chop” the flow supply before it reaches the nozzle, resulting in interruptions to the flow and discrete jet pulses exiting the nozzle. An alternative valving mechanism for generating jet pulses is described in U.S. Pat. Nos. 4,077,569 and 4,863,101. In this method a pressurized flow source is directed into a plenum with a specialized preloaded valve system in which the fluid pressure causes the valve to open, releasing a fluid pulse. When the pressure drops upon opening of the valve, the loading on the valve induces it to close and the flow ceases until the cycle repeats.
A further method for generating jet pulsations, described in Wilson and Paxson (2002) and in U.S. Pat. Nos. 5,495,903, 4,681,264, and 4,389,071, is to use specially shaped channels such as Helmholtz resonators, organ pipes, or resonance tubes to establish and amplify natural fluid oscillations upstream of the flow exit. The resulting oscillations lead to jet pulsation at the flow exit.
An additional method for generating jet pulses is described in U.S. Pat. No. 6,868,790 and utilizes a combustion reaction to drive fluid out of a nozzle in finite bursts of duration related to the burn time of the combustion reaction.
In the prior art described above, the method of pulsation is either the primary means for driving the flow (as with piston-operated pulsation methods) or it is used in series with the primary flow (as in the cases where pulsation is achieved by interrupting the flow). None of the methods described above attempt to keep the primary flow steady (constant) while redirecting it to different nozzle outlets to generate jet pulses as described in the present invention. Generating jet pulses by interrupting or inducing oscillations in the primary flow leads to large pressure fluctuations in the flow and/or requires large storage plenums to properly drive the flow. Using piston-displacement methods to generate jet pulses tends to be inefficient as time and energy are expended retracting the piston and/or refilling the plenum. Hence, in applications such as propulsion it may be preferable to use the method of the present invention so that the advantages of jet pulsation may be gleaned while keeping the primary jet flow substantially steady, thereby allowing efficient and simple means such as ducted fans/propellers to generate the primary flow.
An additional method for generating jet pulses using a valve mechanism is described in U.S. Pat. No. 4,267,856. The method utilizes a single jet inlet and multiple jet outlets with a freely moving obstruction (typically a rubber sphere). The obstruction alternately blocks each of the outlets for a brief period, halting (and hence, pulsing) the flow from that orifice, but no method is provided for controlling the frequency or duration of jet pulses.
U.S. Pat. No. 4,681,264 describes a method for generating jet pulses using a fluid oscillator valve. In this method, pulses are generated by alternately directing inlet flow to two different outlets using pressure feedback loops connected to the primary flow conduit that direct the flow to the respective outlets in time intervals associated with the propagation of pressure pulses through the loops.